Re: [PATCH v3] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 7/10/26 20:03, Matthew Wilcox wrote:
> On Fri, Jul 10, 2026 at 02:34:37PM +0000, Salvatore Dipietro wrote:
>> Commit 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
>> introduced high-order folio allocations in the iomap buffered write
>> path.
> 
> https://lore.kernel.org/linux-mm/aeZzP6iQel-tkZOu@casper.infradead.org/
> 
> I just had a go at implementing what I thought might be the right design
> (having a folio_alloc_orders(min, max, gfp)), but that's not really what
> __filemap_get_folio_mpol() wants because it needs to integrate the actual
> adding of folios to the page cache into the retry loop.
> 
> So instead, let's try this.  The idea is that we want to try direct reclaim
> _twice_.  Once gently (ie with NORETRY specified) when we're trying to
> allocate the maximum order folio.  But now that we've tried that once,
> there's no point trying direct reclaim for other sizes, we just want to
> ask the page allocator if it can give us memory of any subsequent size.
> 
> Until we come to the minimum order.  Then we want to try exactly as hard
> as we were originally asked to try.  So revert to the original gfp flags
> and don't set the NOWARN or NORETRY flags.
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 58eb9d240643..23eecaf9b328 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1983,6 +1983,7 @@ struct folio *__filemap_get_folio_mpol(struct address_space *mapping,
>  	if (!folio && (fgp_flags & FGP_CREAT)) {
>  		unsigned int min_order = mapping_min_folio_order(mapping);
>  		unsigned int order = max(min_order, FGF_GET_ORDER(fgp_flags));
> +		gfp_t alloc_gfp = gfp | __GFP_NORETRY | __GFP_NOWARN;
>  		int err;
>  		index = mapping_align_index(mapping, index);

Note here fgp_flags are checked and may alter gfp towards NOFS or NOWAIT or
add __GFP_WRITE. But alloc_gfp set above will ignore all that for >min_order
attempts, which isn't right. Probably doesn't affect Salvatore's evaluation
though.

> @@ -2004,12 +2005,11 @@ struct folio *__filemap_get_folio_mpol(struct address_space *mapping,
>  			order = __ffs(index);
>  
>  		do {
> -			gfp_t alloc_gfp = gfp;
> -
>  			err = -ENOMEM;
> -			if (order > min_order)
> -				alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;
> +			if (order == min_order)
> +				alloc_gfp = gfp;
>  			folio = filemap_alloc_folio(alloc_gfp, order, policy);
> +			alloc_gfp &= ~__GFP_DIRECT_RECLAIM;
>  			if (!folio)
>  				continue;
>